Conventional digital video encoders and decoders have problems implementing efficient conversion. In particular, efficient conversion from a motion vector difference (MVD) into a binarized representation appropriate for input to a binary arithmetic encoder for compression of the motion vector difference is difficult with conventional decoders. In particular, most conventional approaches separate the binarization of the absolute value of the motion vector difference from the binarization of the sign of the motion vector difference. Different contexts (i.e., statistical assumptions) apply for efficient entropy encoding of the absolute value and the sign.
A first conventional approach binarizes and encodes the sign only for non-zero motion vector differences (MVD). Such an approach puts the sign bit after the first bit of the MVD. The presence or absence of the sign bit can be inferred immediately from the value of the MVD.
Such an approach has the disadvantage that parsing of the absolute value of the MVD is interrupted for the parsing of the sign bit. Such parsing potentially increases the complexity of the binarization and de-binarization processes. Such an approach was implemented in the software version of the H.264 standard in order to get the best possible bitrate, regardless of parsing complexity.
Another conventional approach binarizes and always encodes the sign bit (even for zero motion vector differences) Such an approach places the sign bit after the entire binarization of the absolute value of the MVD. The parsing of the absolute value of the MVD is not interrupted by the parsing of the sign bit.
Such an approach has the disadvantage that extra bits must be sent in the compressed bitstream. Such an approach was described in the JVT-F100draft2b of the H.264 standard specification in order to get the least possible complexity in the binarization process.
It would be desirable to implement a system that (i) only binarizes the sign for non-zero MVDs and (ii) places the sign binarized (if it is coded) after the end of the binarization of the absolute value of the MVD.